I’ve spent a long time studying vacuum tube circuits, specifically audio amplifiers. Vacuum tube amplifiers are generally speaking, pretty simple in nature. Over the years, manufacturers have tried designing very sophisticated preamps, tightly regulated power supplies, various tone equalizer circuits, and fine-tuned the power amplifier sections with various degrees of success. Most of these design changes do a good job of actually improving vacuum tube amplification in some way. However, most of the improvement is in the pursuit of better numbers, including the price, but not necessarily the sound in your ears! That’s why I decided to build the 6DJ8 Headphone Amplifier. It’s a single tube amplifying the signal without anything else in the way. I would describe the 6DJ8 Headphone Amplifier design as elegant. It is clean, simple and smart. If it’s tube sound you want, this is it!
This amp was inspired by Bruce Heran’s, Do-It-Yourself, 6DJ8 Headphone Amplifier Project. There were several things I liked about this amp. It’s single-stage design did not use any coupling capacitors that can “color” the sound of the amp. No negative feedback is used. It uses current production E88CC (6DJ8) tubes, and there are plenty of new-old-stock (NOS) 6DJ8 tube options available on the market as well. I liked the emphasis on the power supply design to provide clean, stable and quiet power to the power amp sections. I liked the idea of using filtered DC on the heater circuit instead of hum-inducing AC. The only thing I didn’t like about his version was his use of solid-state diodes (or a bridge rectifier) on the B+ supply. I was concerned about the noise from the diodes, even “low-noise” diodes, tainting this clean power supply. I also thought about cathode stripping as the B+ hits the cold tubes right after power-on, and this can strip the cathode coating, shortening the life of the tube.
Then I came across Matt, at CascadeTubes.com who built one of these amps. He used a current production, EZ81 (6CA4) tube rectifier for the B+ supply instead of diodes. The rest of the circuit was the same.
For my version, I tried to combined the best of both worlds. I borrowed Matt’s EZ81, tube rectified B+ design with Bruce’s solid-state DC heater supply.
For the heater circuit, the bridge rectifier and two 4700µF capacitors on the DC heater supply provide clean, filtered 6.2V DC to the heaters of the tubes. 6.2V is just within the 6.3V specification for 6 volt tubes and is perfect for getting them warmed up quickly and safely. For the B+ supply, the slower warm-up time for the EZ81 tube rectifier to start conducting means no cathode-stripping DC will hit your expensive output tubes before they warmed up and ready to handle it. All of this means this amp will provide optimum conditions for your expensive 6DJ8 (E88CC) tubes to operate in, both extending the life of the tubes and increasing the performance of the amp.
A note about the bridge rectifier: The original schematic called for a 2 amp bridge rectifier, which would be fine for powering only two tubes. Adding the tube rectifier added another 1000mA of current consumption to the heater circuit with a total current drain of ~1.8 amps! I don’t care for running parts at their maximum so I used a 10 amp part in my build. My bridge rectifier requires a heat sink at 3 amps or better. Even though I’m only pulling just shy of 2 amps, my sample still runs quite hot! I put a decent little heat sink on it to help keep it cool. I chose to omit the heater supply voltage regulator found in Bruce’s original plans. While it’s not a bad idea, I didn’t feel the need to regulate the heater supply.
I designed the whole amp, chassis, wood base, and schematic using LibreOffice Draw. I then ran my designs through one of the best engineers I know: my dad! He was able to double-check and troubleshoot the design with me, and then re-create the drawings in AutoCAD for manufacture.
The chassis is 1/16″ brushed stainless steel. It’s pretty to look at, as well as rigid enough to handle the weight of all the transformers. The stainless steel chassis will also last forever and never rust, and it shines up quite well.
To go with a good design, you need good parts. This amp is so clean and simple, the few parts it has are really going to count!
The original plans called for Edcor power and output transformers. I’m already a fan of Edcor so no argument there! Unfortunately, I couldn’t find a suitable Edcor choke so I’m using a Triad choke instead. All power supply filter caps are Sprague Atom (blue) capacitors with the exception of the heater supply filter caps. Those are 16V Nichicon Black capacitors. All power resistors are wire-wound and rated 10 watts. I also sprung for the excellent quality Alps Blue Velvet volume control.
A few notes about parts: I feel I should have used 1% tolerance resistors in the whole amp even though they are more expensive. I have found that even a few volts difference in plate voltages between output tubes *can* make a difference in the output volume in that channel and lead to an unbalanced stereo image. Most tube vendors offer tube balancing for free or a small extra charge. If you are ordering new production tubes, be sure to get them balanced. If you are ordering new-old-stock (NOS) tubes, have the seller check and balance the pair as close as possible. This will help keep the stereo image balanced properly.
I read through the forums of the original project and learned that transformer placement, especially the power transformer, was crucial to keeping noise and AC hum out of the amplifier circuit and ultimately out of your headphones! The trade-off is the overall size of the amp. It was suggested that a minimum of 7″ should be kept between the power transformer and the output transformers to keep things quiet. I choose to put the power transformer as far back as possible and centered in the chassis, while the output transformers are pinned as far front and towards the edges of the chassis as possible, giving me roughly 7″ of space between power and output. Also, the output transformers are rotated 90° to the power transformer to isolate them even more. All the noisy, high voltage AC is in the rear-center of the layout including the IEC power connector, fuse, and power switch. It’s all much quieter DC and filtering going forward from there.
Results – The Good:
- While it’s not going to earn any fancy art design awards, I think it looks pretty decent! All screws are stainless steel to match the brushed stainless steel chassis.
- The custom wood base really shines and can be made with any hardwood and stained to any color. It could even painted!
- The impedance switch to select between the 32Ω and 150Ω output transformer taps helps find the best match to your headphones.
- It uses current production tubes so there are plenty of new stock out there at cheap to reasonable prices.
- There are plenty of NOS (New Old Stock) tube options available on the market at various prices.
- This amplifier does not use any coupling capacitors in the signal chain to color the sound of the amp. This means that your tube choice is all that matters in the overall sound character, allowing anyone to fine-tune the amp to their specific taste and headphone choice!
- It’s a solid performer and will run until you shut it off.
- The amp sounds absolutely amazing no matter what tubes I use! The background is completely black with only a small amount of hiss with the volume wide open and no input signal.
Results – The Not So Good:
- The Edcor power transformer hums in the chassis a bit, as most Edcor transformers do. With headphones on, you don’t hear it at all so it’s not a real big issue.
- There’s no balance control so you must get matched output tubes or you *might* notice one channel is louder than the other. As well, make sure that you get the same voltages on all the output tube plates!
- On my build, there is no physical protection for the tubes sitting in their sockets on the top of the amp deck. I personally like it this way and think it adds a nice visual to the amp. But, these tubes get very hot and can burn anyone who doesn’t know better and tries to touch one. This is not for toddlers!
- The amp uses only a single gain stage and therefore, only offers a *decent* amount of gain. This amp will get plenty loud enough to drive most headphones out there. However, there are some sources that just don’t offer enough output voltage out of the headphone jack to sufficiently drive the amp and can leave you wanting more gain. Keep this in mind. I have a 7″ Android tablet that simply cannot drive this amp well enough. To be clear, my Android tablet doesn’t get very loud using it’s own speakers. The rest of the sources I’ve tried like the Nexus 6p, the iPhone 4S, 5S, & 6, the Google Pixel 2, and a Sony CD player are absolutely fine.
Special thanks to Mike Amos for hand-crafting this beautiful wood base for the amp!